The present invention relates to a ball bearing, and particularly a ball bearing used to support a rotary shaft, which is disposed in a fan motor of an electric cleaner for domestic use or in a blower of an air conditioner for domestic use and is to be rotated at a high speed with a low load, in such a manner that the rotary shaft can be rotated freely with respect to a housing.
Conventionally, such a ball bearing 1 as shown in FIG. 11 is widely used to support a rotary shaft, which is disposed in various apparatus, in such a manner that it can be freely rotated with respect to a housing. The ball bearing 1 comprises an outer ring 3 including on the inner peripheral surface thereof a deep-groove type of outer ring raceway 2 having an arc-shaped section, an inner ring 5 including on the outer peripheral surface thereof an inner ring raceway 4 having an arc-shaped section, and a plurality of balls 6 respectively interposed between the outer and inner ring raceways 2 and 4 so as to be free to roll; and, the outer ring 3, inner ring 5 and balls 6 are all made of bearing steel such as SUJ2 or M50, ceramic, or the like. The balls 6 are respectively held by a retainer 7 in such a manner that they are able to roll while they are spaced from one another. Also, to the inner peripheral surfaces of the two end portions of the outer ring 3, there are secured the outer peripheral edge portions of sealed rings 8 and 8, whereas the inner peripheral edge portions of the sealed rings 8 and 8 are respectively disposed so as to be close and opposed to the outer peripheral surfaces of the two end portions of the inner ring 5.
By the way, in the case of the conventional ball bearing 1, generally, where the diameter of the respective balls 6 is expressed as Db, the radius of curvature of the section shape of the outer ring raceway 2 is expressed as Roxe2x80x2, and the radius of curvature of the section shape of the inner ring raceway 4 is expressed as Rixe2x80x2, the following equations are established; that is, 0.50 less than Roxe2x80x2/Dbxe2x89xa60.53, and 0.50 less than Rixe2x80x2/Dbxe2x89xa60.52. Also, where the outside diameter of the outer ring 3 is expressed as D, the inside diameter of the inner ring 5 is expressed as d, and the pitch circle diameter (P.C.D.) of the respective balls 6 is expressed as Dpxe2x80x2, the following equation is established; that is, Dpxe2x80x2≈(D+d)/2. In other words, there is employed the equation, that is, Dpxe2x80x2/(D+d)/2≈1, and the respective balls 6 are positioned substantially in the middle of the outer peripheral surface of the outer ring 3 and the inner peripheral surface of the inner ring 5 with respect to the diameter direction of the ball bearing 1.
In case where the above-structured ball bearing 1 is used to support, for example, the rotary shaft of a fan motor disposed in a suction device employed in an electric cleaner, the outer ring 3 is inserted and fixed to a fixed housing, while the inner ring 5 is outserted and fixed to the rotary shaft.
The above-mentioned conventional ball bearing 1 has a general-purpose structure which aims for assembly into one of various rotation support portions, but does not prefer to apply under the low-load and high-speed rotation condition, and, therefore, the rotation torque (rotation resistance) thereof is not always low. On the other hand, there has been increasing a demand for reducing the rotary torque of the rotation support portion in order to be able to cope with a rising energy saving tendency in recent years. In view of such circumstances, it is an urgent need to realize a ball bearing which not only provides a small rotation torque but also can be incorporated into the rotation support portion which rotates at a high speed with a low load. As the simplest means for reducing the rotation torque, it can be expected that, as grease to be applied to the portion where the balls 6 are disposed, grease having low viscosity is used. However, there is a limit to the torque reduction that can be realized by reducing the viscosity of the grease and, therefore, in order to be able to realize large torque reduction, it is necessary to change the structure of the ball bearing itself.
In case where the rotation torque of the rotation support portion rotating at a high speed with a low load is reduced by changing the specifications of the ball bearing, use of a ball bearing whose diameter and diameter-associated elements are reduced in size (that is, a small-sized ball bearing) can realize rather large torque reduction. However, in this case, it is necessary to reduce the inside diameter of a housing into which the outer ring is inserted and fixed, which unfavorably requires the design change of the remaining component members of the rotation support portion. Also, even in case where the diameter and its associated elements of the ball bearing are simply reduced in size, there still remains a possibility that sufficient torque reduction cannot be realized.
The present invention aims at eliminating the above-mentioned drawbacks found in the conventional ball bearing.
Accordingly, it is an object of the invention to provide a ball bearing which not only can realize a low torque structure but also can be assembled to a housing similar to the conventional ball bearing.
In attaining the above object, according to a first aspect of the invention, there is provided a ball bearing which, similarly to the above-mentioned conventional ball bearing, comprises an outer ring including on the inner peripheral surface thereof an outer ring raceway having an arc-shaped section; an inner ring including on the outer peripheral surface thereof an inner ring raceway having an arc-shaped section; and, a plurality of balls respectively and interposed rollably between the outer and inner ring raceways.
Especially, in the ball bearing according to the invention, where the outside diameter of the outer ring is expressed as D, the inside diameter of the inner ring is expressed as d, the pitch circle diameter of the respective balls is expressed as Dp, the diameter of the groove bottom of an inner ring raceway whose maximum circumferential stress provides (294 MPa) 30 kgf/mm2 under the condition that, in case where d is in the range 6-10 mm, the interference of the inner ring is 11 xcexcm and in case where d in the range of more than 10 mm up to 18 mm, the interference of the inner ring is 12 xcexcm, is expressed as Di, x=Db/{(Dxe2x88x92d)/2}, and y=Dp/{(D+d)/2}, the following equations (1) to (2) can be satisfied, and also the following equation (3) can be preferably satisfied: that is,
xxe2x89xa70.3xe2x80x83xe2x80x83(1)
y less than 1.0xe2x80x83xe2x80x83(2)
yxe2x89xa7{(Dxe2x88x92d)/(D+d)}x+2Di/(D+d)xe2x80x83xe2x80x83(3).
Also, preferably, where the diameter of the respective balls is expressed as Db, the radius of curvature of the section shape of the outer ring raceway is expressed as Ro, and the radius of curvature of the section shape of the inner ring raceway is expressed as Ri, the following equations (4) and (5) can be satisfied: that is,
0.53 less than Ro/Dbxe2x89xa60.65xe2x80x83xe2x80x83(4)
0.52 less than Ri/Dbxe2x89xa60.65xe2x80x83xe2x80x83(5).
In the case of the above-structured ball bearing according to the first aspect of the invention, not only sufficient durability can be secured but also sufficient rotation torque reduction can be realized without changing the outside diameter of the outer ring specially.
That is, in order to satisfy the equation (2), the plurality of balls are positioned on the inside diameter side of the ball bearing. This can reduce the moment necessary to roll these balls, therebybeing able to reduce the rotation torque of the ball bearing.
In this manner, even when reducing the rotation torque of the ball bearing, in order to satisfy the equation (1), by securing the diameter Db of the balls, the contact ellipses in the contact portions between the balls and outer ring raceway can be prevented from decreasing in size excessively, which can prevent Brinell impressions from occurring in the outer ring raceway.
Further, in order to satisfy the equation (3), by securing the pitch circle diameter Dp of the balls, even when the inner ring is outserted onto the rotary shaft, circumstantial stress occurring in the inner ring can be prevented from increasing excessively, which can prevent the inner ring against damage such as occurrence of a crack.
By the way, in the equation (3), the inner ring raceway (groove bottom) surface Di depends on the fit standard js5 specified in JIS and on the strength that is required of the inner ring. That is, according to the js5, the upper limit value of the interference of an inner ring is 11 xcexcm in the case of an inner ring having an inside diameter of 6-10 mm and, in the case of an inner ring having an inside diameter of 10-18 mm, it is 12 xcexcm.
Further, the outer ring, the inner ring and the plurality of balls are preferably made of bearing steel. Generally, the inner ring raceway surface Di having an influence on the thickness of the groove bottom of the inner ring is specified in such a manner that the maximum stress of bearing steel can be of 137.2 MPa (14 kgf/mm2) or less. However, actually, depending on the selection of the material of the inner ring and on the change of the thermal treatment thereof, up to the stress of 294 Mpa (30 kgf/mm2), the thickness of the groove bottom can be reduced. For this reason, the inner ring raceway surface Di is set at the value where the maximum circumstantial stress provides 294 MPa (30 kgf/mm2).
By the way, referring to the ratio R1/Db between the diameter Db of each ball and the radius of curvature R1 of the section shape of the inner ring raceway, as in a fan motor used in a blower of an air conditioning apparatus, when the fan motor is used at a speed of 10000 minxe2x88x921 (r.p.m.) or less, the ratio is set such that 0.52 less than R1/Dbxe2x89xa60.65; but, as in a fan motor used in a suction device of an electric cleaner, when the fan motor is used at a speed of 20000 minxe2x88x921 (r.p.m.) or more, preferably, the ratio may be set such that 0.53 less than R1/Dbxe2x89xa60.65.
Moreover, by satisfying the above equations (4) and (5), contact ellipses, which are formed in the contact portions between the rolling surfaces of the balls and the outer ring and inner ring raceways can be reduced in size so that rolling resistance and spin, which are caused in the contact ellipse portions during rotation, can be reduced to thereby be able to reduce the rotation torque of the ball bearing. By the way, in case where the values of Ro/Db and Ri/Db exceed 0.65 and increase excessively, the area of each of the contact ellipses is reduced excessively, which makes it difficult to secure the rolling fatigue lives of the outer and inner ring raceways; and, especially, in the case of the outer ring raceway, Brinell impressions are easy to occur. For these reasons, the upper limit values of Ro/Db and Ri/Db are set at 0.65.
In attaining the above object, according to a second aspect of the invention, there is provided a ball bearing for use in an electric cleaner which comprises an outer ring made of bearing steel and including on the inner peripheral surface thereof an outer ring raceway having an arc-shaped section; an inner ring made of bearing steel and including on the outer peripheral surface thereof an inner ring raceway having an arc-shaped section; and, a plurality of balls respectively made of bearing steel and interposed rollably between the outer and inner ring raceways.
And, the present electric cleaner ball bearing is incorporated into the rotation support portion of the electric cleaner and is used in such a manner that the outer ring is fixed and the inner ring is rotated at the speed of 40,000-60,000 minxe2x88x921 (r.p.m).
Especially, in the ball bearing according to the second aspect of the invention, where the diameter of the respective balls is expressed as Db, the radius of curvature of the section shape of the outer ring raceway is expressed as Ro, and the radius of curvature of the section shape of the inner ring raceway is expressed as Ri, the following equations (1) and (2) can be satisfied: that is,
0.58xe2x89xa6Ro/Dbxe2x89xa60.61xe2x80x83xe2x80x83(1)
0.52xe2x89xa6Ri/Dbxe2x89xa60.61xe2x80x83xe2x80x83(2)
In the case of the above-structured ball bearing for an electric cleaner according to the second aspect of the invention, not only sufficient durability can be secured but also sufficient rotation torque reduction can be realized without reducing the outside diameter of the outer ring specially.
That is, by satisfying the above equations (1) and (2), contact ellipses, which are formed in the contact portions between the rolling surfaces of the balls and the outer ring and inner ring raceways, can be reduced in size so that rolling resistance and spin, which are caused in the contact ellipse portions during rotation, can be reduced to thereby be able to reduce the rotation torque of the ball bearing.
By the way, the reason why, as described above, the ratios of the radius of curvature of the section shape of the outer ring raceway Ro and the radius of curvature of the section shape of the inner ring raceway Ri to the diameter of the respective balls Db are respectively set in the range of 58-61% is as follows. That is, as these ratios increase, the contact ellipses formed in the respective contact portions decrease in size, thereby being able to reduce the rotation torque of the ball bearing. Therefore, in order to reduce the rotation torque of the ball bearing, it is preferred to increase these ratios (that is, Ro/Db and Ri/Db) . On the other hand, in case where these ratios are increased, the surface pressures of the respective contact portions increase, which lowers the exfoliation lives of the outer ring raceway and inner ring raceways. Here, FIG. 10 shows the relation between the above ratios and the exfoliation lives of the outer and inner raceways under the operation conditions {rotation speed=60,000 minxe2x88x921 (r.p.m.), and preload of 49 N (5 kgf)} of a ordinary electric cleaner ball bearing (the outside diameter D of an outer ring=22 mm, the inside diameter d of an inner ring=8 mm, and the width B of the ball bearing=7 mm).
As can be seen clearly from FIG. 10, generally, when the durability of the rotation support portion is taken into account, it is not expedient to form outer and inner ring raceways having such large radiuses of curvature that provide the ratios (that is, Ro/Db and Ri/Db) of more than 56%. On the other hand, as in an electric cleaner ball bearing to which the present invention relates, when a ball bearing is used under the conditions that the rotation portion is rotated at a high speed with a low load and dust such as brush friction powder can invade into the interior portion of the ball bearing, the life of the ball bearing depends, in many cases, on the occurrence of seizure rather than on the coming of the exfoliation life. And, due to the enhanced speed of the rotation of the rotation support portion, in many cases, such seizure occur in the range of 2,000-3,000 hours. Therefore, it is no expedient that, in order to obtain an exfoliation life which exceeds greatly 2,000-3,000 hours, the above ratios are reduced (that is, the ratios are approximated to 50%), because this increases the rotation torque of the ball bearing. When such circumstances are taken into consideration, in case where the above ratios are respectively set in the range of 58-61%, not only a practically sufficient exfoliation life can be secured but also the rotation torque of the ball bearing can be reduced to a sufficient degree.